Conventionally, there has been a system for detecting a deflated tires based on the rotation speed (wheel speed) sensor information. This system uses a principle according to which deflated tires show a reduced outer diameter (tire dynamic loaded radius) compared to that of tires with a normal air pressure (reference pressure) and thus show an increased rotation speed and an increased rotation angle speed compared to those of other normal pressure tires. In case of a method of detecting deflated tires based on a relative difference in the rotational angular velocity of tires (see Patent Literature 1 for example) for example, a determination value DEL obtained byDEL={(F1+F4)/2−(F2+F3)/2}/{(F1+F2+F3+F4)/4}×100(%)is used. In the formula, F1 to F4 represent rotational angular velocities of a left front tire, aright front tire, a left rear tire, and a right rear tire, respectively.
By the way, a tire dynamic loaded radius is influenced by factors other than a decreased pressure (e.g., tire load shift during cornering). Thus, in order to correctly determine deflated tires, a decreased pressure must be determined based on the tire rotation information that is obtained when the vehicle is not cornering (i.e., that is obtained during going straight). This also applies to system calibration when the tire air pressure is adjusted to a normal pressure or a tire is exchanged with a new one. Thus, calibration must be performed based on the tire rotation information during going straight.
Patent Literature 2 discloses, in order to improve the calibration accuracy as described above, a technique to monitor a vehicle lateral acceleration (lateral G) and a steering angle to perform calibration based only on the rotational angular velocity data that is obtained when the lateral acceleration and the steering angle are within a predetermined range.